I think in order to answer your question it is necessary to start with the different sonnet forms.

English (or Shakespearean) Sonnet - probably the most common studied in high school literature; 14 lines made up of 3 quatrains and a rhyming couplet at the end; typical rhyme scheme ABAB CDCD EFEF GG or ABBA CDDC EFFE GG; written in iambic pentameter. Examples: look at Shakespeare

Italian (or Petrarchan) Sonnet - 14 lines made up of one octave (8 lines) and one sestet (6 lines); the octave's rhyme scheme typically goes A B B A A B B A; the sestet's rhyme scheme usually has two or three rhyming sounds made up in a number of ways (C D C D C D; C D D C D C; C D E C D E); iambic pentameter. Examples: look at Wordsworth

The Spenserian Sonnet - 14 lines made up of 3 quatrains and a rhyming couplet at the end; rhyme scheme looks like ABAB BCBC CDCD EE; iambic pentameter. Examples: look at Edmund Spenser

The Indefinables - typically is 14 lines with some sort of rhyme scheme and attempts iambic pentameter but might be very loose. Examples: any 14 lined poems that rhyme could be considered a sonnet.